A conventional bicycle chain drive sprocket consists of a sprocket mounted upon the bicycle crank having a hub portion, and a plurality of chain engaging teeth defined on the sprocket rim periphery. Such sprockets are widely used in single speed, three speed and ten speed bicycles. As the number of teeth formed in the sprocket periphery determines the speed transmission ratio between the crank sprocket and driven sprocket affixed to the rear wheel, the size of the crank sprocket, and the number of chain engaging teeth defined thereon, is predetermined to provide the desired speed ratio.
A form of bicycle racing has become popular commonly known as BMX racing utilizing a single speed bicycle of relatively small size which is pedaled over a variable terrain course. As such bicycles are of a single speed the transmission ratio between the drive sprocket and wheel sprocket can only be varied by the installation of different sized chain sprockets and it is common for the riders to change the size of crank drive sprocket rims to accomodate a particular race course, the size and strength of the rider, or for other reasons, and BMX riders normally carry several sizes of sprockets for use as desired.
To reduce the time and effort required to change the size of the crank sprocket rim on a bicycle manufacturers of BMX drive sprockets have produced a variety of sprocket peripheries of various size which may be mounted upon or detached from a hub permanently affixed to the bicycle crank. The hub is provided with threaded inserts, or bolt receiving holes, and includes locating surfaces thereon for centering the chain sprocket upon the hub and the fasteners will maintain the engagement of the locating surfaces defined upon the sprocket and hub.
As the locating surfaces and inner configuration of the various manufacturers of demountable sprocket rims differ from each other it has heretofore not been readily possible for the sprockets of different manufacturers to be mounted upon a single hub, and previously, the drive ratios available to a rider having a hub of a single manufacturer were limited to those ratios of chain sprockets manufactured by the particular manufacturer of the hub. As each manufacturer does not usually produce sprockets with single teeth differences the rider who wanted a drive sprocket which has only one tooth more or less than the sprocket being used would have to use a sprocket rim of a different manufacturer requiring the installation of a new hub upon the crank, which is a time consuming procedure. Accordingly, as a practical matter, a rider is limited to the speed ratios available from the manufacturer of the hub installed upon his bicycle, and while the several manufacturers of this type of demountable sprocket jointly produce a wide range of sprocket rim sizes varying only by a single tooth the rider's availability of such various sizes of sprockets of various manufacturers was limited by the requirement for installing a hub upon the crank to accomodate the sprocket of a particular manufacturer.
Demountable chain sprockets have been available for many years as shown in U.S. Pat. Nos. 1,433,923, and 2,436,694, and such demountable sprockets may use segmented tooth portions, or the sprocket may constitute a single piece, both types being shown in the aforementioned patents. Further, it is known to form a bicycle chain sprocket of synthetic plastic material and of such configuration as to be mountable upon a variety of crank spiders as shown in U.S. Pat. No. 4,144,733, and such a construction permits a single size sprocket to be mounted upon various types of bicycle crank spiders.
It is an object of the invention to provide a universal bicycle sprocket hub which may be mounted upon the bicycle crank and is capable of accomodating demountable chain sprocket rims of the major manufacturers of such sprockets whereby the sprockets of such manufacturers may be mounted upon a single hub thereby making available to the rider a selection of speed ratios available from a variety of sources.
A further object of the invention is to provide a universal bicycle sprocket hub upon which locating surfaces are mounted which are capable of accurately centering a plurality of constructions of demountable sprockets thereon, and wherein the sprockets may be quickly mounted or removed from the hub by means of threaded fasteners.
Another object of the invention is to provide a universal bicycle sprocket hub which is formed of a synthetic plastic material, is of an economical construction, of high strength, and which may be molded of attractive colors.
The bicycle sprocket hub of the invention is preferably molded of a high strength synthetic plastic and includes a central opening through which the bicycle crank is received and a crank pin opening receives the crank pin. A first radially extending locating surface is defined upon the hub adjacent its periphery and a plurality of axially extending shoulder surfaces intersect the first locating surface, the configuration of the shoulder surfaces including radially extending portions equally circumferentially annularly disposed about the hub axis. A second radially extending locating surface is inwardly disposed of the first locating surface, and two sets of axially extending shoulder surfaces intersect the second locating surface also defining radially extending portions equally annularly disposed about the hub axis.
A plurality of fastener receiving holes are defined in the hub extending in the axial direction and intersecting the locating surfaces whereby bolts, threaded inserts, or similar threaded fasteners are used to affix the chain sprockets to the locating surfaces, and the configuration of the sprockets is such that a flat surface thereof will be held against the associated hub locating surface.
The sprocket hub is reinforced on its inner side by a plurality of radially extending ribs, and reinforcement means are defined adjacent the crank pin opening for reinforcing the hub against crank pin driving forces.